MOSAIC Imaging Performance

Overview

Imaging performance was assessed at the J and K bands during the night of UT 1996 September 21. The H-band filter was defective and broke during the cool-down cycle (it had survived 4 previous cool-down cycles). A new H-band filter is on order and will be installed in October.

Image performance was assessed using observations of stars in the open cluster NGC 7790 for which the USNO has accurate astrometry and previous IR photometry to assess the photometric performance of the ALADDIN array. The night was clear and photometric, with 1.2-arcsec seeing reported at WIYN during the night. Typical best images at the 2.1-meter with MOSAIC were 0.8-arcsec FHWM (see below).

There is a separate page with pretty pictures taken during the run.


Plate Scale

Fred Vrba and Arne Henden (USNO) reduced and analyzed the images of NGC 7790 to determine the plate scale and rotation. The solutions are based on 34 stars detected on the MOSAIC frames which are in common with the UJ1.3 catalog produced by Dave Monet's PMM (Plate Measuring Machine) program at USNO.

KPNO 2.1-meter (f/7.8 secondary):

These give a total field of view of ~175x349-arcsec (2.9x5.8-arcmin) with the half-ALADDIN array.

Future calculations with a more complex model should allow measurement of higher-order distortions if present.

NOTE: The MDM 2.4-meter plate scale will be measured during the upcoming October run.


Image Quality

MOSAIC produces superb images at J and K bands (the H-band filter was defective and broke this run and will be replaced in October). The two images below show stellar images at J and K (in this an all subsequent images, click on the thumbnail picture for a full-size view).

The fundamental limitation on image quality was the failure of the primary mirror support for the 2.1-meter mirror between the J and K frames. The airbags apparently deflated, causing the mirror to set down on its hard points. This results in trefoil shaped image seen in the K-band frame (where we should get our best atmospheric seeing).


Left: J-band image of a star in NGC 7790; Right: K-band image.
Images are single 10- and 5-second integrations, respectively.

Seeing at J and K was 0.8-arcsec FWHM and 0.9-arcsec FHWM, respectively. The latter larger due to problems with primary mirror support. Below are radial profiles of the star images above:


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Updated: 1996 September 26 [isl]